1. Field of the Invention
The present invention relates to lithotripsy equipment and, more particularly, to an improved electrode design that can be universally adapted for use with a range of lithotripters of various manufacturers.
2. Description of the Background
A lithotripter is a device that pulverizes kidney stones and gallstones by passing shock waves through a water-filled membrane that presses against the side of the patient. Extracorporeal shockwave lithotripters (ESWLs) in particular are used for treating kidney and biliary stones. The first ESWL lithotripter was developed in West Germany, and the US Food and Drug Administration (FDA) approved its use in the United States in Dec. 1984. Since then hundreds of thousands of patients have been treated. Lithotripters can use a number of methods of generating shock waves. Most typically, shock waves are generated by an electrode or “spark plug” placed at the focus of an ellipsoidal reflector. The spark from the plug vaporizes a small amount of water, creates a shock wave, and the ellipsoid reflector focuses each shock wave to a point about half a foot above it. A bombardment of successive shock waves has been found effective at disintegrating many stones including kidney stones.
The spark plug electrodes are usually constructed with an inner conductor which is surrounded by an insulating layer. The inner conductor extends beyond the insulation to an electrode tip. An opposing second electrode tip is spaced from the first electrode tip to provide a spark gap there between. A cage surrounds the electrodes and provides a conductor and necessary structure.
Examples of electrodes appear in several U.S. patents.
U.S. Pat. No. 5,105,801 to Cathignol et al. (Technomed) suggests that decreasing the resistance of the water decreases the latency time of the shockwave and actually increases the acoustic pressure.
U.S. Pat. No. 5,251,614 to Cathignol et al. discloses a lithotryptor electrode with closely-spaced discharge electrodes forming part of a discharge circuit having an inductance L and a capacitance C defining a critical resistance Rc equal to the square root of (L/C),
U.S. Pat. No. 5,195,508 to Muller et al. (Dornier Medizintechnik) issued Mar. 23, 1993 shows a spark gap unit for lithotripsy with a pencil conductor with an inner electrode, and insulation that envelops the pencil conductor. An external cage conductor is formed with a bow and an outer electrode. The patent illustrates a hollow inner space inside the insulation of the pencil conductor, the space being open rearwardly for easy placement of a current-feeding plug (connected to the inner electrode).
U.S. Pat. No. 4,905,673 to Pimiskern issued Mar. 6, 1990 (Dornier System GmbH) shows a lithotripsy probe with an inner and an outer conductor with electrode tips. The two electrodes have tips of initially different diameter, the tips being flattened (truncated cones) and facing each other, the diameter of inner electrode being initially larger than the diameter of the tip of the outer electrode.
U.S. Pat. No. 6,217,531 to Reitmajer (ITS Medical Technologies & Services GmbH) issued Apr. 17, 2001 shows an adjustable electrode that self-measures the discharge voltage, compares it to a reference voltage, issues a correction signal, and operates an adjusting mechanism that repositions the electrodes, thus optimizing the spark gap.
U.S. Pat. No. 5,047,685 to Nowacki et al. issued Sep. 10, 1991 shows an electrode structure for lithotripters having inwardly turned tips with spaced confronting faces lying on opposite sides of the axis of the reflector.
Extracorporeal lithotripters are quite expensive, typically between $300,000 to $550,000, and their spark plug electrodes such as the foregoing are also expensive components. The rapid and frequent discharges of energy across the electrode tips has been found to erode and/or deteriorate the electrode tips, and replacement is often required.
U.S. Pat. No. 5,420,473 to Thomas issued May. 30, 1995 shows a partial solution in the form of a spark gap electrode assembly for lithotripters that allows easy replacement of both electrode tips without requiring manual adjustment of the spacing between the tips.
U.S. Pat. No. 6,849,994 to White et al. (Healthtronics) issued Feb. 1, 2005 is very similar to the above-noted '473 patent to Howard. Specifically, it shows an electrode assembly for lithotripters with a pencil conductor removably connected to an insulating layer. External threads on the pencil conductor cooperate with internal threads in a bore of the insulating layer to fixably secure the insulating layer in a desired position relative to the inner conductor and discharge electrode tip. FIG. 1 is an exploded view of the prior art '994 White et al. device. This spark plug-type electrode assembly 10 included an inner conductor 12 having an insulating layer 22 inserted thereon. A discharge tip 26 is inserted into the inner conductor 12 and extends from opening 30 at the distal end 32 of the insulating layer 22. A housing 34 has an internal bore 36 which allows the housing 34 to be disposed about the exterior surface 38 of the insulating layer 22. The housing is equipped with a plastic clip 40 that connects to an electrical power connection in the lithotripter. In this and other prior art electrodes the clip 40 is keyed to the lithotripter. The housing 34 is joined to a cage base 50 which serves as an outer conductor, conducting electricity through arms 52 to upper tip holder 54 which receives the second electrode tip 56. The cage base 50 surrounds bore 36 which extends over the insulating layer 22 as well as the upper housing 44, and its arms 52 are spaced apart providing access to a spark gap, which is the space directly between the electrode tips 26, 56. Accordingly, when a spark is generated, the acoustic shock waves may be transmitted from the spark gap through a reflector, and on through the tissue of a patient to break up the stones.
The '473 Thomas and White '994 patents suggests a partial solution to the problem in the form of a spark plug electrode having electrode tips that can be easily replaced. Nevertheless, with these and other known spark plug electrodes the electrode is keyed to the lithotripter and is available only as original equipment dedicated to a particular manufacturer's equipment. Spark plug electrodes from different manufacturers are not interchangeable. This effectively prevents replacement or substitution of the entire spark plug electrode assembly and compels purchase of an original equipment replacement
Accordingly an improved electrode design is needed to allow adaptation to numerous lithotripters from various manufacturers.